WO 03/00480 (Novo Nordisk A/S and Boehringer Ingleheim International GMBH) describes a series of substituted piperazines and diazepanes as H3 antagonists. WO 02/08221 (Neurogen Corporation) describes a series of substituted piperazines and diazepanes as capsaicin receptor antagonists which are claimed to be useful in the treatment of neuropathic pain. WO 98/37077 and WO 99/42107 (Zymogenetics Inc) both describe a series of substituted heterocyclic derivatives which are claimed to act as calcitonin mimics to enhance bone formation.
The histamine H3 receptor is predominantly expressed in the mammalian central nervous system (CNS), with minimal expression in peripheral tissues except on some sympathetic nerves (Leurs et al., (1998), Trends Pharmacol. Sci. 19, 177-183). Activation of H3 receptors by selective agonists or histamine results in the inhibition of neurotransmitter release from a variety of different nerve populations, including histaminergic and cholinergic neurons (Schlicker et al., (1994), Fundam. Clin. Pharmacol. 8, 128-137). Additionally, in vitro and in vivo studies have shown that H3 antagonists can facilitate neurotransmitter release in brain areas such as the cerebral cortex and hippocampus, relevant to cognition (Onodera et al., (1998), In: The Histamine H3 receptor, ed Leurs and Timmerman, pp 255-267, Elsevier Science B.V.). Moreover, a number of reports in the literature have demonstrated the cognitive enhancing properties of H3 antagonists (e.g. thioperamide, clobenpropit, ciproxifan and GT-2331) in rodent models including the five choice task, object recognition, elevated plus maze, acquisition of novel task and passive avoidance (Giovanni et al., (1999), Behav. Brain Res. 104, 147-155). These data suggest that novel H3 antagonists and/or inverse agonists such as the current series could be useful for the treatment of cognitive impairments in neurological diseases such as Alzheimer's disease and related neurodegenerative disorders.